1. Field of the Invention
This invention relates to photodetectors and particularly to an improved detector having an extremely fast response to light pulses.
2. Description of the Prior Art
Present biplanar coaxial photodiodes utilize a transparent anode mesh on the tube faceplate and a parallel photocathode spaced from the anode with an accelerating potential of about 2 to 5 Kv across the gap. Use of higher electric field gradients is limited, since higher potentials cause breakdowns. The photocathode electron emissive coating is spaced from the faceplate to reflect electrons toward the anode. This avoids a voltage drop which would occur as a result of the series resistance of the coating if it were disposed on the faceplate in the direct path of light passing through. Where the finite resistance of the photocathode is not a problem, the coating may be disposed on the input faceplate and the anode spaced therefrom. The photocathode of the biplanar tube provides the inner conductor of a high frequency coaxial output transmission line and the anode provides the outer concentric tubular conductor. An example of such a prior art tube is found in an article in the Journal of Physics and Scientific Instruments, Vol. 8, 1975, printed in Great Britain, entitled "Temporal response and real time measurements with a 5 GHz photocell-oscilloscope system at low light levels", by B. Sipp et al.
The tube response time to light pulses is limited by the voltage and the relatively large gap between the anode and photocathode. In addition, a high voltage capacitor is normally required in series with the photocathode inner coaxial line to block the high voltage from the low impedance instruments or amplifiers that are coupled as a load to the output of the phototube. This capacitor causes additional difficulties with undesired oscillations or ringing losses, and possible breakdown. There are also problems with the vacuum seal which is spaced along the line from the photocathode-anode gap and causes relatively slow decay time, impedance mismatch, and pulse reflections.